When you are called to the bosses office (since supposedly, they are the only ones who have carpet) and its definitely not for a good reason, i.e., you are in trouble, something has not gone according to plan and either maybe you are responsible and/or have some explaining to do.
This idiom is used when describing the way a situation appears, while allowing for the possibility that things may be different:
On the face of it, the company looks very profitable. (The company appears to be very profitable, but this may not be the case.)
(UK) When a horse runs, its head moves backwards and forwards alternately - in horse racing, if 2 horses cross the line together the one whose head happens to be going forward often wins and is said to win 'on the nod'.